1. Technical Field
The present invention relates to a polarized light communication device which employs polarized laser light as a transmission signal, and, in particular, to a polarized light communication device optimally employed for communication between a strongly dispersing medium, such as the human body, and the external environment. The present invention further relates to a reflected light detector optimally employed when obtaining information relating to the dispersing medium's flow, etc., and to a pulse wave detecting device which uses this detector to obtain the pulse wave in the body.
2. Background of the Invention
In conventional wireless communication, radio waves are typically employed to carry out communication. However, wireless data communications with faster forwarding rates are needed, and new frequencies must be developed. In addition, progress has been made in the radio wave field toward realizing the practical application of semi-millimeter and millimeter waves.
At the same time, light, which is not classified as radio waves under the law, has been increasingly used for wireless communications. In wireless data communications employing light, it is possible to offer high speed data communications using a broad band not regulated as radio waves. Since light characteristically does not pass through non-transparent objects such as walls, it is suitable for short-distance data communications, or wireless LAN systems confined to one room. Currently, the most representative methods from among wireless communication methods using infrared are the IrDA (Infrared Data Association) types of infrared data communication function. These consists of an infrared light emitting diode and a light receiving element, and can realize data conversion at a speed of from 115.2 kbps to 4 Mbps. Although this technology has a short communication distance of 1 m or less, its major feature is its ability to provide wireless data communications at low cost.
There is need to develop from now optical wireless data communications capable of a larger forwarding capacity and a longer communication distance. However, when using a light emitting diode as the light source, a problem arises with respect to the effective utilization of the band since the light emitted from the light emitting diode has a wavelength width of 100 nm or more. Furthermore, since the LED is limited by the carrier life, modulation above 100 MHz is difficult. In order to resolve these problems, it is effective to use a semiconductor laser as the light source. If a semiconductor laser is employed, it becomes easy to obtain a wavelength width of 1 nm or less, while modulation of 1 GHz or more is possible in principle. However, erroneous operations caused by crossing may be a problem.
Unlike radio waves, light used as wireless carrier waves is not legally regulated. Thus, while light can be freely employed, interference between optical wireless devices employing the same wavelength may occur. For example, the IrDA method, which is a known form of optical wireless data communications, employs wavelengths in the range of 850 to 900 nm for the peak wavelength. Thus, while a semiconductor laser may be used to realize a communication device for long-distance, high-speed communications, interference will occur with the IrDA method when any wavelength in this 850 to 900 nm range is employed. The IrDA method is widely employed in computers currently in use. Accordingly, even if interference with the IrDA method used in these computers does not present a problem from a legal perspective, it must be avoided from the perspective of practical application.
In the medical field, a variety of sensors are used to detect the body's internal status in order to continually monitor an afflicted area. The ability to record and analyze phenomena occurring inside the body is extremely important both for clarifying physiological functions and for diagnosing and treating a variety of illnesses. A number of methods have been investigated for this purpose. When invasive conventional methods are used to directly measure signals generated inside the body, then a problem arises in that the measurements must be conducted at the hospital bedside. Conversely, when an attempt is made to conduct measurements of physiological phenomena under a natural environment in which the user is engaging in daily activities, then an indirect method of measurement must be employed. Thus, the signals within the body cannot be taken directly. Accordingly, with the objective of directly measuring the signals generated inside the body under a natural environment, an arrangement may be conceived in which the entire component needed for directly measuring the signal inside the body for a computer and measuring circuit is embedded in the body, such that the device is autonomously completed within the body. In this case, however, the method of communication between the device within the body and a device outside the body becomes a problem.
For example, if communication is carried out using wires, then not only is infection a concern, but the device may hinder the user's daily activities. Furthermore, if radio waves are used, then there is a chance that radio waves generated by other communication devices may have an effect. Moreover, in addition to wireless communication devices, electromagnetic waves are emitted by electronic devices or thunder, raising the possibility of erroneous operations being cause by these electromagnetic waves. In addition, radio waves can maintain their SN ratio when propagating over long distances, raising the problem of interception of or interference from the radio waves from another person. This type of problem occurs not only in the case of devices which perform physiological measurements, but also in the case of such physiological assisting means as pacemakers, artificial kidneys, or insulin pumps. In other words, when sending a monitoring signal from a physiological assisting means embedded in the body to a device outside the body, or when sending a control signal from the device outside the body to the physiological assisting means, communication must be carried out between the two devices. The problems which may occur in this case are exactly the same as those discussed above.
Therefore, in order to avoid the above-described problems, an approach may be considered in which a light emitting diode is used to strongly modulate light (infrared light). This strongly modulated light is then employed to carry out communication between the inside and the outside of the body (see The Institute of Electronics, Information and Communication Engineers Shinshu University Technical Journal October 1995, MBE 95–89). Specifically, as shown in FIG. 46, an infrared transmitting and receiving circuit may be connected to a serial interface between computer systems 131,134, located within and outside the body respectively. At the infrared transmitting and receiving signal circuit, transmitting circuits 132,136 transmit digital data output from the serial interface as infrared light. The infrared light received by receiving circuits 133,135 is converted to digital data and relayed to the computers. A CPU, memory, real-time clock with calendar, A/D converter, or serial interface for communication with the outside may be employed for computer system 131 inside the body, and may be miniaturized to about the size of a business card through surface mounting technology. Using the above-described device, it is possible to eliminate the problems encountered when using radio waves.
However, it has not been possible to realize full duplex communication when employing light as the transmission signal for communicating between the inside and the outside of the body. This point will be explained below.
FIG. 45 is an example of a communication device for sending strongly modulated light as a transmission signal between the inside and the outside of the body. Physiological function assisting means 201 is embedded inside the body. Transmitter 211, provided to physiological function assisting means 201, emits strongly modulated light a by controlling the amount of light emitted by the light emitting diode. Receiver 202 of control means 222 which is outside the body receives light a emitted from transmitter 211 of physiological function assisting means 201. Meanwhile, receiver 212 of physiological function assisting means 201 receives strongly modulated light (not shown) emitted from transmitter 221 of external control means 202.
However, the body consists of a medium in which dispersion is extremely large (strongly dispersing medium), and is complexly formed of such sources of dispersion as body fluids, cells, and tissues. Therefore, light a preceding through the body is gradually dispersed in a variety of directions. As a result, as shown in FIG. 14, a portion of light a emitted by transmitter 211 of the internal assisting means reaches receiver 212 of internal assisting means 201. As a result, when internal transmitter 211 internal receiver 212 is sending a light signal, it is not able to receive light signals. In other words, only half duplex communication, in which only unidirectional communications can be performed simultaneously, is realized. Thus, it was not possible to realize full duplex communication in which communication in both the sending and receiving directions can be performed simultaneously.
Full duplex communication is the required method in the case where urgent controls or warnings may be required. For example, an environmental change may occur as a physiological function assisting means is sending data on physiological measurement data, such that urgent control of the physiological function assisting means must be provided from the outside. If full duplex communication cannot be carried out in this case, then even if an attempt is made to send commands or data to the device inside the body from the transmitter on the device outside the body, it is necessary to wait until transmission and receipt of the aforementioned measured data is completed. Since it may be urgent that control of the device inside the body be performed, the delay of the transmission or receipt operation constitutes a serious problem. Thus, full duplex communication are a necessity.
In addition, in a communications signal which passes through a strongly dispersing medium such as the body, the quantity of light which is received by the receiver is a very small proportion with respect to the quantity of light emitted by the transmitter. In order to compensate for this, it is necessary to sufficiently increase the amount of light emitted. Thus, a large amount of electrical power is required. When transmitting from outside to inside the body, the outside transmitter is able to use a large amount of electric power. In contrast, there is a limit to the amount of electric power which can be used by the device inside the body during transmitting from within to outside the body. Thus, in view of practical use, it is not desirable that the transmitter consume a large amount of power.
Moreover, it has been reported that when using a light emitting diode as the light source, considerable damping occurs upon passage through the skin (see The Institute of Electronics, Information and Communication Engineers, MBE-97-5 “Dermal optical telemetry system using laser diode”, IJO, et al).
A device for detecting the radial arterial wave is available as one example of a conventional device for detecting pulse waves. In this type of device, changes in pressure at the skin surface in the vicinity of the radial artery are detected using a pressure sensor. The pulse wave is measured in this way. Since changes in the pressure applied to the sensor placed on the surface of the skin over the radial artery are detected, it is necessary to apply a pressing force of 30 to 80 mmHg in order to carry out a stable pulse wave detection. Accordingly, this has been problematic since the pressure felt by the test subject is large.
For example, in the invention disclosed in U.S. Pat. No. 4,951,679, a pressure sensor disposed near the redial artery is pressed against the arm, the pressing force is then sequentially varied, and the pressing force at which the amplitude of the detected signal is greatest is detected. The pulse wave detection is then carried out at this pressing force. In this case, the optimal pressing force can be detected so that the application of a greater than necessary force can be avoided. Nevertheless, however, it is still necessary to apply a specific amount of force on the arm, so that the subject still feels a strong pressure sensation.
In contrast, examples of pulse wave detecting devices which do not require the application of a strong pressing force include devices using ultrasonic waves or light (infrared, laser, etc.). In pulse wave detecting devices using ultrasonic wave reflection, the pulse wave is measured by bringing a probe emitting ultrasonic waves into contact with the user's arm from the outside, and then receiving the ultrasonic waves reflected by arterial vessels and the like at the probe.
In pulse wave detecting devices which detect the pulse wave using light, light is sent from a light emitting diode into the body, and the amount of light reflected (light reflected by subdermal tissues, etc.) is detected. A portion of the light emitted from the light emitting diode in this case is absorbed by the hemoglobin in the blood vessels. Thus, the amount of reflected light is related to the amount of blood in the blood vessels, and is detected as the pulse wave.
In conventional pulse wave detecting devices employing ultrasonic waves, the value detected for the reflected wave will vary according to the angle formed between the blood flow and the probe where the ultrasonic wave is sent and received. It is difficult to maintain the probe at a fixed angle with respect to blood flow during operation, however, so that a stable pulse wave measurement is difficult to achieve. For example, when the probe is in contact with the user's arm on the palm side, it becomes difficult to detect the pulse wave if the position of the probe is displaced by just a few millimeters with respect to the arterial vessels. Moreover, when the probe is placed in contact with the back of the user's arm, then it is not possible to maintain the S/N required for detecting the pulse wave.
In addition, even in a device which employs a laser or light emitting diode, it is difficult to discriminate between the attributes such as wavelength, phase, or degree of polarization which belong to the reflected light, and the attributes which belong to natural light or various illuminating lights. As a result, an impact from natural or illuminating light tends to be present, so that stable and accurate detection of the pulse wave becomes problematic.
For example, a method is conventionally known in which the dispersing medium is irradiated with light (electromagnetic waves), the reflected light is detected at the light receiving element, and temporal changes in the flow quantity of the dispersing medium are detected. Note that “dispersing medium” as used here means a substance having the property of dispersing iremitted light, and includes not only fluids and flows containing a mixture of microparticles, but also bodies such as the human body. In the case of the body, the body is irradiated with light, and a light receiving element then detects the light which is reflected. As a result, information, such as pulse wave information, relating to the body can be obtained. This method is particularly significant because detection of the pulse wave can be carried out in a non-invasive manner.
In a method in which a dispersing medium is irradiated with light, the reflected light is detected, and information relating to the dispersing medium is obtained, if the light receiving element detects not only the reflected light component, but also the external light component, then it is not possible to accurately obtain information relating to the dispersing medium. Accordingly, the important technique in this method is the reduction of the impact from external light. External light is typically of an extremely strong intensity such as sunlight, or light in which the intensity has been modulated to a commercial frequency, as in the case of florescent lights. Moreover, it should be noted that even if the intensity of the external light is constant, the intensity of the external light component detected by the light receiving element will change if the light receiving component is moved.
If the light quantity which is emitted on the dispersing medium is increased to an extent such that the influence of the external light can be ignored, then this problem can be resolved at once. However, increasing the light quantity is not practical when one takes into consideration the properties of the light emitting element which emits the light, the amount of power it consumes, and safety with respect to the body when obtaining information relating thereto. Accordingly, it will be necessary to consider this problem below, with the assumption that there is an upper limit to the amount of light emitted.
Based on this assumption, in order to reduce the influence of external light, one may first consider using a filter to remove unnecessary wavelengths components from the light detected by the light receiving element. If a semiconductor laser is used, then light in a narrow wavelength band can be emitted. Therefore, if a glass filter which transmits only light of this wavelength band is disposed in front of the light receiving component, it should be possible to reduce the influence of the external light.
In order to reduce the influence of the external light, a second method may be considered in which, after taking into consideration the properties of the dispersing medium, the wavelength of the light employed is selected to be in a band in which the effect of external light is not readily imparted. For example, light in the infrared region readily passes through the body, while light having a short wavelength, such as blue light, is readily absorbed. Therefore, when obtaining information related to the body, a blue light LED is used as the light source, while a photodiode using GaP or GaAs which is sensitive to the blue light region is used as the light receiving element. As a result, it is possible to decrease the impact of external light.
However, the first method, employing the glass filter, has the following problems. Namely, it is not possible to realize sharp characteristics such as those when transmitting only the wavelength band of the semiconductor laser. While such characteristics can be realized with an interference filter, the production cost thereof is typically high. The characteristics of the transmission wavelength band must be matched to the semiconductor laser used. Thus, costs rise considerably.
On the other hand, in the second method, in which the wavelength of the light is selected to be in a range which is not readily influenced by external light, a suitable light source and light receiving element may not necessarily be available depending on the wavelength of the selected band. For example, when obtaining information relating to the body, a blue LED and a photo diode sensitive to blue light are employed. However, in general, these devices are not only expensive, but they consume a large amount of power and are poorly efficient with respect to photoelectric conversion. In addition, the fact that blue light is readily absorbed by the body is a positive effect with respect to reducing the influence of external light, but a negative effect with respect to its difficulty in reaching deep areas in the skin. For this reason, when obtaining information relating to deep areas under the skin, a large amount of light is needed, contradicting the assumption stated above.
Particular problems exist when obtaining information related to the body. These include the superimposition of a motion component should the body move, so that the information obtained is not accurate, or a marked deterioration in detection sensitivity when the air temperature is low due to contraction of the capillary vessels at the skin surface.